In a typical thermal printer, a web-type carrier containing a repeating series of spaced frames of different colored heat transferable dyes is spooled on a carrier supply spool. The carrier is paid out from the supply spool and rewound on a take-up spool. The carrier moves through a nip formed between a thermal print head and a dye-absorbing receiver. The receiver is in turn supported by a platen in the form of a drum. The print head engages the dye carrier and presses it against the receiver. The receiver may, for example, be coated paper and the print head is formed of, a plurality of heating elements. When a particular heating element is energized, it produces heat. In the presence of heat and pressure, dye from the carrier is caused to transfer to the receiver. The density or darkness of the printed color dye is a function of the energy delivered from the heating element to the carrier. These types of thermal printers offer the advantages of "true continuous tone" dye density transfer. This result is obtained by varying the energy applied to each heating element, yielding a variable dye density image pixel on the receiver.
The web-type carrier often includes a repeating series of yellow, magenta and cyan dye frames. The carrier is typically formed of a very thin, flexible dye carrying member having a thickness that can be on the order of 1/4 mil. At the beginning of the print cycle, the head must be lifted off the drum to allow a receiver sheet to be wrapped about the drum and advanced under the print head. This pre-printing process requires that the drum turn without the head or carrier being in contact with the drum. To begin printing, the first dye frame, typically yellow, is advanced to a position under the print head. The print head is lowered by a head loading mechanism to apply pressure on the carrier-receiver (media) as the drum turns. The media slides under the print head and the heating elements are selectively energized to form a row of yellow image pixels under the print head. The drum turns to generate successive rows of the yellow pixels of the final image. When the yellow image has been deposited, the head is lifted and the receiver is repositioned for the next color frame of the carrier. During such repositioning, the carrier is moved so that the next dye frame, for example magenta, is positioned under the print head. When the printer is ready for the second dye frame, the head is lowered to re-establish contact with the media, and the next color image is deposited on top of the previous color image in the receiver. The process is repeated for the final color, in this case the cyan dye frame. The three dyes are blended during the deposition process to generate a full-color image. After the three colors of the full-color image have been deposited, the printing process is completed. The head must be lifted again to allow the drum to turn and eject the receiver. The print head must continue to be held up for the next receiver sheet.
The process of applying the print head to the drum must be done in a manner that allows the head to be positioned accurately, repeatedly, and with uniform pressure across the drum to provide a high-quality print. The thermal head linear array of heating elements should be positioned tangent to the drum and centered radially over the drum surface. In addition, the heating elements should be pressed against the platen surface with uniform force across the receiver surface. Because manufactured parts vary from perfect dimensions, a head loading mechanism should be designed to minimize the effect of these dimensional errors on print quality. If this accuracy cannot be built into the head loading mechanism, adjustments must be built in. Such adjustments add to the complexity and expense of the mechanism. The repeatability of the mechanism is guaranteed if the print head returns to the same position after a lift-and-lower cycle. If the print head does not return to the same position for each of the dye frames, the resolution of the image will be degraded.